text:
How do gyroscopic instruments work?
I'm trying to better my understanding of the physics behind gyroscopic instruments in an aircraft. I am aware that a spinning gyroscope creates rigidity in space by resisting applied forces. But, what exactly is the resistive force acting on the externally applied to forces, and how is it created? Is that what precession is?

I'm voting to close this question as off-topic. This question would be better on Physics.SE The answer is inertia in the spinning plan (conservation of angular momentum).

Yeah your probably right. Didn't cross my mind, I just came straight here.

I saw this pretty cool video a while ago:

It gives an intuitive discussion of gyroscopes.
On another level though, it boils down to the angular momentum of the spinning gyroscope, and the fact that in order to change that momentum's direction/magnitude, an external moment must be applied.

Gyroscopes basically work by the principle of conservation of angular momentum. Consider a gyroscope (or a spinning wheel) in space like shown below:

Image from pigeonsnest.co.uk
Consider the wheel in the center spinning at an angular velocity $\omega$, so that the spin angular momentum is $I\omega$, along the X axis. Now, consider a torque applied $\mathcal{T}$ along the Z axis.
Now, the resultant of these two makes the spin axis to align along the axis A'. Basically, the spin angular momentum 'chases' the torque in the same plane. This is the basis of gyroscopic precession.
See Prof. Walter Lewin's lecture on Rolling motion and gyroscopes for a thorough explanation.


text:
How do plane landing-gear tyres roll before take off?
How do plane tyres roll? Are they equipped with special engines like cars or are they rolling using the plane flying engines? Or can both methods be found with different constructors?

Welcome to the site! Related: How does an aircraft taxi?, also  Does it make sense towing airplanes to the head of airstrip by external means? and last What is the cost savings of using electronic motors to taxi?

Normally, aircraft wheels are rotating freely around their axle, unless the brakes are activated. The ground roll was a convenient time for engine warm-up in the days of piston aircraft.
However, rolling on the ground using jet engines is very inefficient; fuel consumption could be reduced if the aircraft uses other means of propulsion on the ground and only starts the engines right before take-off. Therefore, electric propulsion using hub motors is actively studied. The batteries could be charged (partially) during landing, when the hub motors are switched to generator mode, and the rest of the power could be provided by the APU.
Pushing back from the gate using jet engines is unwise, so a tug is used to move the aircraft to a position from which it can continue the ground roll to its take-off position. These tugs push a bar which is linked to the nose wheel of the aircraft. Towbarless (TBL) tugs scoop up the nosewheel and lift it off the ground, allowing more-secure control and faster maneuvering.

Conventional tug Goldhofer F110E (picture source)

Boeing 777 being pulled by Goldhofer TBL aircraft tug (picture source). They are really hard to see when you are sitting in the cabin.

Towbarless tug detail (picture source)

It is actually possible for jet aircraft to push themselves back from the gate using thrust reverse, but it is just a stupid idea to do.

@SMSvonderTann a stupid idea to do - it's standard practise at many regional airports.

@Simon Ah, you might want to edit this question then because it says: "Pushing back with jet engines is impossible" right above the images.

It's called a power back and it can be done. Here is a DC9 doing it and even some prop GA aircraft like the Aviat Husky can reverse by putting the prop in beta

@RonBeyer: Yes, corrected. But power backs are highly controversial. And you did note that I was speaking of jets, not turboprops, didn't you?

@PeterKämpf Yes, although I'm not sure why you mention it, the Husky is a light GA aircraft with a generic piston engine if that is what you are referring to. The DC9 is certainly equipped with Jet engines...

During take-off roll, the aircraft tyres move due to the thrust generated by the engines (or propellers for that matter). They don't have any engines (motors) fitted inside them to help in movement. In case the engine is not producing thrust, the aircraft are towed using a tug.

I sure I saw Bombardier Dash 8 walking its way to the runway (I was in) with the propellers OFF and without any tug

@MakhloufGharbi The Dash 8 cannot move without a tug, or it's engines.

I was in, so perhaps I didint seen the tug :) . I'm not sure

Some aircraft actually do have motors inside the wheels, but this technology is still in a testing/demonstration phase.  EGTS and WheelTug are the two competing products under development.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
What does the Boeing 777 autopilot do after reaching the last programmed waypoint?
As I understand it, a series of waypoints can be programmed, by a pilot, into one of the autopilots on a Boeing 777. If the autopilot is engaged, the autopilot will cause the aircraft to fly to each waypoint in turn, automatically steering the aircraft as necessary.
It is not clear to me whether the autopilot automatically disengages after reaching the final waypoint or whether it maintains (for example) a magnetic heading and altitude.
What happens after reaching the last programmed waypoint if the pilot does not intervene?

In an attempt to rescue this question from closure, I have replaced the original question with one simple generic question that I believe addresses your main concern. If you dislike what I have done you can undo my change by clicking the "edited" link and then "rollback".

The first point I would stress on is that you don't program the autopilot. You program the FMS (Flight Management System), which looks like a very large calculator. In this picture it is the device under then first officer's hand.

On top of the PFD (Primary Flight Display) you see the MCP (Mode Control Panel). This is what controls the modes that the autopilot system will follow, here is another image of it:

Typically the entire flight plan is programmed into the FMC and then during various stages of flight, the autopilot is engaged to help with navigation.
This flight plan includes the departure, enroute waypoints or tracks, and then generally the arrival at the destination; but that is it - it does not include the runway information, etc (as this may be subject to change).
The FMC takes this information and other data (weight, fuel, temperature, weather, etc.) to calculate a flight plan which is then displayed; and the pilot will then select and activate a flight plan.
In most commercial flights however, the flight plan is already loaded in the FMC from the flight operations department and the pilots simply select the predefined flight plan. Punching in a large flight plan takes a significant amount of time so many times this is already downloaded from the flight ops center.
At any stage of flight, the autopilot may be armed and engaged to do one of many things - either maintain thurst, or heading, or control the climb/descent; or follow the flight plan. It may also be disarmed during any stage of flight.
If the autopilot is armed and it set to follow the FMC's flight plan; then after the last waypoint it will follow the waypoint heading.
So, if the last way point is a holding pattern, it will continue on that pattern. If the last waypoint was a marker it will reach that marker, and then continue at that heading and speed; even though there are no more commands coming to it from the FMC (since its at the end of the programmed flight plan).
Note that reaching the end of the flight plan from the FMC does not disengage the autopilot; there are many other things that might do that though.
All the above comes from my time in a 737 and 777 simulator.

One nitpick.  The MCP programs the flight director and if the autopilot is engaged, it follows the guidance from the flight director (which is also displayed on the PFD regardless of autopilot activation).

The last waypoint is usually at a holding pattern where the plane will fly in circles until the pilot disengages to continue to land.
For example in Helios flight 522 the crew got incapacitated and the plane ended up in a holding pattern for an hour until it ran out of fuel. This was a Boeing 737 but I imagine the 777 having the same.

This really depends...  A lot (most) flight plans that commercial jets fly end either with an arrival or the actual airport when there are no arrivals.  The end of an arrival may be a holding pattern, but is much more often a specific heading or an instruction to expect radar vectors after the last fix.

When a Boeing Jetliner reaches the end of the programmed flight plan track with the autopilot engaged in Lateral NAV and the pilot does not intervene, (which is usually at a terminal aid (VOR or NDB) at the destination airport); the autopilot will remain engaged, however the tracking mode will default to the last flown track heading into the last navigation aid and the heading window will open showing the numerals.  Pilot intervention is then required to change the heading or tracking of the aircraft as desired or instructed by ATC.

do you have any reference?

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How Realistic (or not) was the crash scene in Flight?
In the movie Flight, Denzel Washington's plane loses one engine and some other controls. To keep the plane gliding, he inverts the plane until it reaches a very low altitude. At the last second he flips it back right side up so that the belly of the aircraft takes the grunt of the crash landing.
Obviously this is all hollywood and takes a lot of creative license, but I'm a bit curious as to how realistic it was. Would you actually be able to fly a large jet upside down like that?

You say "loses one engine", which implies that the plane has more than just one. A plane is only gliding if it has no engines running at all.

It's apparently loosely based on Alaska Airlines Flight 261 and like in the actual incident, I don't think he deliberately inverts the plane. In the actual event, the horizontal stabilizer gets jammed in an extreme position thus forcing the plane to invert. Now if it were possible to fly a commercial airplane that big inverted, Alaska Airlines too would've landed safely eventually (which didn't happen, despite the pilots trying to fly it upside down according to the CVR transcripts).

For the record, in the movie he did intentionally invert the airplane.

I'm not familiar with the movie but the scenario described is nonsense. An aircraft that is in good enough shape to perform aerobatics is in pretty damned good shape and could just continue with its flight and perform a normal landing.
Some large aircraft are capable of brief inverted flight. Tex Johnston famously barrel-rolled the Boeing 707 prototype in 1965 (Wikipedia, Youtube). The Alenia Spartan, a Hercules-sized turboprop transport, is also capable of doing barrel rolls and often does so at air shows (Youtube).

a barrel roll is a positive G maneuver which means that inside the plane the (apparent) gravity is still to the bottom of the plane, sustained inverted flight is much harder and unless the fuel tanks are setup for it will starve the engines at the very least

The concept in the movie is that the horizontal stabilizer jack screw failed, which resulted in it fail ing to the full nose down position, with no way to control it. They then entered a steep dive and rolled inverted so that they would climb instead of descend. They weren't just doing random aerobatic maneuvers. :)

Downvoted on the basis that you haven't seen the film, and therefore don't know what is being discussed here. The topic directly relates to the film, and their depiction of a crash caused by a failed horizontal stabilizer jack screw. It is therefore not an aircraft in "pretty damned good shape".


text:
What are the FAA requirements for an aircraft to be IFR certified?
As I understand it, the FAA certifies certain aircraft types for IFR flight in general. All aircraft of that type are then certified, not something you have to do with each individual aircraft.
What are the minimum requirements for aircraft to be certified for IFR? Is it all location-sensing equipment?
And a bonus: can I get a single aircraft that is only VFR certified to be IFR certified if I add more equipment?

As you say, the FAA certifies an aircraft for certain "kinds of operation".  For a normal category airplane certified under 14 CFR 23, it basically says that it must be "established appropriate to the installed equipment":

§23.1525   Kinds of operation.
  The kinds of operation authorized (e.g. VFR, IFR, day or night) and the meteorological conditions (e.g.
  icing) to which the operation of the airplane is limited or from which
  it is prohibited, must be established appropriate to the installed
  equipment.

The installed equipment required for IFR flight (beyond that required for VFR flight)  is listed in 91.205(d) and is mostly equipment used to communicate with ATC and to maintain control of the airplane when flying without visual reference to the ground (so no navigation equipment is specified):

§91.205   Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
...
(d) Instrument flight rules. For IFR flight, the following instruments
  and equipment are required:
(1) Instruments and equipment specified in paragraph (b) of this
  section, and, for night flight, instruments and equipment specified in
  paragraph (c) of this section.
(2) Two-way radio communication and navigation equipment suitable for
  the route to be flown.
(3) Gyroscopic rate-of-turn indicator, except on the following
  aircraft:
(i) Airplanes with a third attitude instrument system usable through
  flight attitudes of 360 degrees of pitch and roll and installed in
  accordance with the instrument requirements prescribed in §121.305(j)
  of this chapter; and
(ii) Rotorcraft with a third attitude instrument system usable through
  flight attitudes of ±80 degrees of pitch and ±120 degrees of roll and
  installed in accordance with §29.1303(g) of this chapter.
(4) Slip-skid indicator.
(5) Sensitive altimeter adjustable for barometric pressure.
(6) A clock displaying hours, minutes, and seconds with a sweep-second
  pointer or digital presentation.
(7) Generator or alternator of adequate capacity.
(8) Gyroscopic pitch and bank indicator (artificial horizon).
(9) Gyroscopic direction indicator (directional gyro or equivalent).

As far as converting an airplane from VFR to IFR, the Type Certificate Data Sheet (TCDS) must specify that the airplane is designed for IFR flight, and then you could add the missing equipment to make it qualify.

"(so no navigation equipment is specified)" - navigation equipment appears to be specified under part (2)? Or do you mean it doesn't specify exactly what navigation equipment is required, only that some is present?

No lighting for instruments required?

@JonStory Not "some", but "suitable for the route to be flown." That part isn't specific because it can be met in various ways.

@Andrius There are different requirements for night flying, which include those.


text:
How does an aircraft taxi?
Does an aircraft use its engines to taxi or is it always pulled by some other vehicle?
I have seen a small lorry pulling it sometimes; is it really pulling the huge thing, or does the aircraft use its own power somehow?

For the single engine airplanes that most of us probably fly, it's quite normal for a single person to push/pull them around by hand.  Often the plane comes with a tow bar that has a handle and hooks on the nose gear.

Aircraft are not always moved with tugs (the small vehicle you refer to). Much of the time, they operate under their own power.
Airliners are generally pushed back from the gate using a tug, as that's the simplest and most efficient method. They then taxi to the runway (and all the way up to the gate, after landing) using their own engines to provide thrust. This is true of both jets and propeller aircraft.
Some work is being done on self-contained electric-powered ground propulsion; one major player in this area is a company called WheelTug. They use high-torque motors and the aircraft APU to allow the aircraft to move itself without starting its engines or requiring a tug to be connected and disconnected.

There was at least one airline back in the 60s or 70s that was powering-back 727s for awhile at some stations.

@Terry yeah, that's true. DC-9s occasionally did this as well (Northwest, maybe?) but it's not very common. But yeah, that's why I didn't say that airliners always get push backs!

AA was doing it into the 90s if memory serves correctly. You have to have an Opspec to do it though. OPSPEC C065, POWERBACK OPERATIONS WITH AIRPLANES.
A.     C065 authorizes the use of powerplant reversing systems for rearward taxi operations...

Yeah, I remember both AA and Northwest doing powerbacks commonly back in the 90s. I guess with the price of fuel, it wasn't as expensive back then as it is now.

@reirab Well, and then there are drive-in jet bridge stands such as some on CDG T1 :) goo.gl/maps/jRMB3BXrRt92

Usually an aircraft taxies under its own power. Aircraft have no powered wheels, so the forward thrust comes from the engines. Only pushback from the gate is mostly done by a tractor, although some aircraft can use reverse thrust for that.
When the aircraft is being moved for maintenance purposes or simply relocation at the airfield, the aircraft is usually pulled.
Research is going on into fitting electro motors into aircraft wheels so that they can taxi without having to start the engines. Lufthansa has done some experiments. The concept would save taxi fuel and is more friendly to the airport environment (noise, polution), however the additional weight of the motor drive system would offset these benefits because it causes additional fuel burn during flight.

taxies under its own power? do you mean its engines or its wheels have some sort of power?

@shabby, "under its own power" means thrust produced either by turbine or propeller, depending on powerplant.

Most aircraft taxi using their own engine power. Tugs are usually only used to get an aircraft to a safe distance away from all structures, people, and other aircraft before engine start, or if the engine(s) are inoperable (maintenance, mothballing, etc.). However, there is a system (called the ElectricGreen Taxiing System, introduced at the 2013 Paris Air Show) that attaches to the wheel and uses an electric motor to move instead of the main engine(s). It is currently only available on the Airbus A320 family and the Boeing 737 family.

I would say more than just "a safe distance away". The idea would be to get the aircraft pointed outward so that it can thenceforth move forward under its own engine power. And aren't engines usually started at the gate anyway? It's just that it's usually not desirable to try to back up under engine power (discussed many times in this site).

Using electric or hydraulic drive on wheels to permit self-powered pushback and taxiing is an interesting idea, but adds weight that has to be justified for other purposes (such as braking during landing?). Unlike a hybrid car, there's not much you can do with energy generated from the wheels. Also, using a tug, visibility to the rear from a tug and ground crew is usually going to be better than from the cockpit, reducing the chances of bumping into another aircraft on the ground during pushback. Maybe rear-view cameras would be installed?

@PhilPerry I think the systems he's referring to here is actually a separate vehicle, not an electric motor that is actually carried on board during flight. Those have been proposed, too, but they suffer from the weight problem that you mention, which is especially problematic on aircraft that spend much more time flying than taxing (such as long-haul aircraft.)

It's not for pushback (well maybe it is but that's not the purpose). It's for taxiing on the taxiways, saving a lot of fuel from not taxiing under engine power.


text:
Is constant air pressure maintained inside the fuselage?
I just heard it somewhere but I'm not sure about it.   If it is, then why don't they build airtight fuselages?

It would be interesting to have answers that differentiate between passenger aircraft, cargo, and military.

For the purposes of air pressure inside aircraft, they are broken down into two broad categories:

Unpressurized - These aircraft are typically piston powered and can't go very high, so do not even attempt to regulate the air pressure inside the fuselage.
Pressurized - These aircraft do maintain pressurization in the fuselage, but it isn't constant beyond a certain point.

The category determines the answer to your question:
Unpressurized Aircraft
Unpressurized aircraft are not built air-tight for a couple of reasons:

It is very hard to make something as large as an airplane truly air-tight, and keep it that way.  Over time, seals will start to leak, fittings will loosen, etc.
The fuselage would have to be much stronger to withstand the pressure difference between the inside and the outside.  This would make them heavier and more expensive.
There isn't much need.  These airplanes don't fly high enough to really require a pressurized fuselage and a person can breathe more or less normally even though you aren't at sea level.  (Think of people that live on mountains.)

Pressurized Aircraft
Without going into the details of how an aircraft is pressurized (which isn't really relevant to your question), the air pressure inside the fuselage of a pressurized aircraft is usually maintained at sea level pressure up to a certain point.  However, the fuselage is designed to withstand a particular maximum differential air pressure, so in order to protect it (basically keep it from "popping" when the air pressure outside the airplane drops too low), the pressure is allowed to decrease when needed.
In typical jet airplanes, the pressure inside portions of the airplane (typically the passenger compartment, some of the cargo holds, and maybe an avionics bay) at cruising altitude is maintained at a pressure which corresponds to an outside air pressure at an altitude of about 5,000 - 8,000 feet (referred to as "cabin altitude"), depending on the cruising altitude and the design of the airplane.

Many aircraft are not pressurized and therefore not airtight. The occupants would suffocate, if nothing else, and most unpressurized aircraft do not regularly operate at altitudes requiring pressurization.
In the case of pressurized aircraft, it's actually necessary that the cabin is not airtight. They are, instead, nearly airtight and can withstand significant pressure differentials.
Cabin pressurization works by bleeding high-pressure air from the engines, which is then run through packs (heat exchangers and air cycle machines) to cool it. After conditioning, the air is fed into the cabin. This is what creates the lower cabin altitude relative to the outside atmosphere.
Because more air is constantly being added to the fuselage, a pressure outflow valve is required and it is constantly dumping air from inside the aircraft. In normal operations, it's automatically controlled, but can be manually overridden in emergency situations.

Great answer, but I'm not sure about suffocating.  People in submarines & mini subs don't seem to suffocate very often.

@DanPichelman: Early diesel subs had to surface regularly to refresh their air supply; modern nuclear boats (and possibly diesels) have onboard plants that scrub CO2 and/or generate oxygen. Minisubs, much like SCUBA divers, carry bottled air and presumably also use scrubbers. Aircraft that were genuinely airtight would need to carry a lot more equipment.

@egid nuclear subs can generate oxygen because the power plant does not use oxygen. Simply using conservation of energy, we can show that it is impossible for a diesel sub to generate enough electricity to generate oxygen. If fact, most diesel subs, don't run the engine under water, but use batteries to conserve oxygen.

I think you will find that virtually all diesel subs in operation today can operate their engines with the hulls submerged, as well as operate on battery power when submerged to a greater depth.  They use a snorkel device pioneered by the Germans in WWII.    This does not change the validity of your answer regarding nuclear subs, though.

Pressurized aircraft are not airtight, and neither are they constant pressure.  One key facet that would make being airtight impossible is the need to change pressure in the cabin.
To change pressure in the cabin, air is pumped in via the packs (covered in another answer), and air is let out via outflow valves.  The input is fairly constant flow and pressurization is modulated by how open/closed the outflow valve is.
Cabin pressure must change because the airframe has a maximum pressure differential between the cabin and outside (7.8 PSI in the EMB-145) and as you climb the pressure in the cabin must be reduced.  The cabin is generally 8000 ft pressure altitude at during cruise.  Subsequently the cabin pressure is increased to the landing airport during descent.
Accompanying these changes in pressure are temperature changes naturally occurring as result of the pressure change (see: ideal gas law).  You may notice this during descents in regional jets as they tend to get very warm as the cabin is compressed.
To regulate the temperature, you need air conditioning, and so you have packs and air cycle machines to deliver air at the proper temperature to the cabin.  Now that you have to provide air into the cabin, its not worth even thinking about CO2 scrubbers and O2 tanks, because the packs take care of that with fresh air.
That should cover airplanes not being airtight, why they aren't airtight and that the also are not constant pressure.

The answers so far have talked about the differences between pressurized and unpressurized airplanes, however to answer your question why they're not airtight:
The answer is two-fold, first as egid pointed out, the pressure differential would probably be too great for the hull to withstand.
Second, and in my opinion, more importantly, humans consume oxygen and output carbon dioxide (and water). Having your ship airtight, would cause everyone inside to slowly but surely suffocate from carbon dioxide poisoning and lack of oxygen as the little oxygen carried within the aircraft slowly gets breathed away. So you need to keep replacing that air, and the easiest way to do that is to let the old air out, and put new air in.
Edit
As Dan Pichelman points out, people in Subs aren't dropping like flies. True, they're not, but they are in fact also replacing the air. Almost. They're removing the CO2 (using Scrubbers) and adding Oxygen (from oxygen tanks). It would probably be possible to solve it this way for aircraft as well. The question remains whether it is feasible weight wise, and if it's easier to just pressurize the aircraft using bleed air and dump the old air overboard.

This is why people in submarines are constantly dying?

@Dan; well yes, if the oxygen tanks they carry along run out, they would most likely die. Airliners carry oxygen tanks as well, but only for the pilots in emergencies.

Sorry, snarky comment on my part - I'll remove in a minute.  My point is how long will it take to consume the O2 given the cabin volume?  If it takes 6 hours to reach dangerous CO2 levels in a plane with 3 hours of fuel...

I'll try to dig up some data, would be interesting to know.

A 737-800 has (less than) 1.6 cubic meters of air per passenger (I assumed square cabin cross section, and used the full length of the aircraft). So let's say about 1500 liters/person, and we inhale about 8 liters/minute of 20% oxygen, and exhale 15% oxygen, if we didn't replace it with CO2, and simply discarded 2 liters of the air it'd last 13 hours with 0 oxygen left afterwards. I'm no physician, but I'd wager you'd die from CO2 poisoning long before that. It CAN probably be solved like with subs, but as in the answer, it's easier to just replace it.

@roe yeah, it's also almost certainly lighter to replace it, and it definitely requires less energy.

Some submarines add oxygen from tanks, but many actually generate it through the electrolysis of water.

@egid; interesting, didn't know that. The first google hit said oxygen tanks.. :) What do they do with the hydrogen? Dump overboard?

@Dan as for why people die in subs,  Hollywood provides no end of material,  but concerning realism nothing can beat Das Boot.

In many General Aviation aircraft (small planes that don't go above approximately 12,000 ft.), the air pressure in the fuselage is the same as the outside air.
Most people have no difficulties at altitudes up to 12,000 ft, so there is no need to maintain a pressurized environment.
Specific rules state that unpressurized airplanes have supplemental oxygen available, depending on how high the plane is, for how long, and whether its flying during the day or night.


text:
What is the NACA Series of the Rockwell B-1b's wing design?
I can't find any information about the airfoil of the Rockwell B-1b. It's listed as a NACA 69-190-2, which doesn't follow the conventional format of the 6-Series airfoils. I'm assuming its a design unique to Rockwell but I thought it would still be listed in the airfoiltools database. I cannot find any information about it. Jane's has a write up in the 86-87 edition but only mentions the leading edge dimensions.
From Jane's:

Wings:
Cantilever low-wing fail-safe blended wing body structure, with variable geometry on outer panels. ...
Each of the outer wing panels, which have 15" of leading-edge sweep when fully forward and 67" 30' when fully swept, is a conventional two-spar aluminium alloy torsion box structure, with machined spars, ribs, and one-piece integrally stiffened top and bottom skin panels.
Full span seven-segment leading-edge slats on each outer panel can be drooped 20" for take-off and landing.
Six-segment single-slotted trailing-edge flaps on
each outer panel, with maximum downward deflection of 40°.

I've retracted my vote to close, since the "why spoilers & not ailerons for roll control" question was separated out. This question is now appropriately focused, IMHO.

It is not listed as NACA ..., it is listed as NA 69-190-2. When the development contract for the B1 was awarded in 1970, North American and Rockwell had already merged. Hence, I am assuming that NA stands for North American and that it is a proprietary airfoil. Since the B1 flies at high subsonic speeds when the wings are extended, I would assume that the airfoil is a supercritical one, which also rules out a NACA 6-series airfoil.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Purpose of CHT drop in piston engine shutdown procedures?
One of the items in the helicopter Robinson R22 (and possibly other piston engined aircraft) shutdown procedure is to wait for the Cylinder Head Temperature to drop to a lower temperature. What is the purpose of this?

Related question, if not a duplicate.

According to this document, this is to allow the engine to cool and level the oil temperature:

CHT Drop- Wait two minutes or until you can clearly see the H on Cyl Hd
  Temp. This is to allow the engine to cool and level the oil
  temperature. Closing down too early may damage the engine.

This is most probably to prevent damage to the engine due to shock cooling. Any rapid decrease in temperature during shutdown can cause damage to cylinder heads However, note that this theory is quite controversial. This is discussed in detail here and here. Especially, as @Lnafziger points out in his answer,

... in fact the engine cools much more quickly when you shut it down at the end of a flight.

I believe an effect of shutting down that way is to allow the cylinder heads to burn off any excess.  Leaving the cylinder heads clean makes for an easier startup the next time around.  In  my opinion it would be equivalent to pulling the mixture to lean.

@user6035379 Usually piston aircraft engines are shut down not using the mag switch but by pulling the mixture knob full lean and letting the engine die of fuel starvation. I'm not sure what you refer to when you say "burn off any excess", it probably isn't fuel.

I was taught that pulling the mixture has a few results.  Besides starving the engine to shutdown it also serves as a cleanser to leave the cylinders clean.

@user6035379 - I have a few dual hours in the R-22. Straight from my R-22 CFI, the R-22 and R-44 need a cool down period after shutdown. We had to run the engine near idle for at least two minutes after each flight, keeping an eye on the CHT. Since the mixture was all the way in during the cool down period, the max amount of fuel for that rpm speed was being fed to the engine at that time. The excess fuel helps to cool an engine. Remember, the R-22 runs at 80-90% RPMs the entire time from startup. Other aircraft have the time it takes to taxi to their tie-downs or hangars to cool down.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is "judgemental oversteering" in taxi procedures?
I found a phrase that I don't understand while reading Jeppesen charts: "judgemental oversteering".
I found it in general information about taxi procedures. As I understand this procedure applies only to jumbo aircraft because it takes place only if the wingspan is more than 118ft (according to the chart which I read).
Can you explain to me what is it? What kind of conditions can require this procedure? Examples or images would be appreciated.

"Judgmental oversteering" means to intentionally not follow the taxiway centerline when turning. It is a technique used on large aircraft to turn on tighter or smaller taxiways.
In a sense, it is like driving a long vehicle, such as a truck or a bus. Instead of following the lines on the road, the driver would deliberately overshoot the entry of turn, i.e. delay turning the steering wheel. This technique allows the rear wheels to stay on the road instead of going over the corner which may have fences, poles, pedestrians or other obstructions.
Here is a drawing from the 747x Quiet Longer Range Family Airport Compatibility document which illustrates the tracks of each gear for the "judgmental oversteering" and "cockpit centerline" methods:

This FAA document about airport design summarizes the pros and cons for each method (p.52 of PDF, p. 34 of document):

Cockpit Over Centerline. Taxiway intersections designed to accommodate cockpit over centerline steering require more pavement, but enable more rapid movement of traffic with minimal risk of aircraft excursions from the pavement surface. (...)
Judgmental Oversteering. Taxiway intersections designed to accommodate the judgmental oversteering method of maneuvering require the least pavement widening. However, judgmental oversteering requires complex maneuvering, increases the risk of aircraft excursions from the pavement surface, and slows the flow of traffic.

"pedestrians or other obstructions" - :)

@Simon Think LXGB :D


text:
How should passengers report SOS signal sightings?
On January 23, 2015, I flew from Buenos Aires, Argentina (SAEZ) to Santiago, Chile (SCEL). It was a very comfortable night flight in a Boeing 787 Dreamliner of LAN airlines, though it was delayed.
During the descent and a pronounced turn to the left at approximately 4,000ft above the Andes Mountains, I saw through the window a wood fire in the middle of darkness with some shadows around. Then a brilliant white light blinking: three short flashes, three long flashes, three short flashes. I thought: Wow! That antenna blinked like an SOS signal!
One second after the first sequence, again: three short, three long, three short. Then I unfastened the security belt to run and tell the closest flight attendant: Sir! There is an SOS signal there, and he started to give attention to it.
After a successful landing on runway 17 L/R, I asked the flight attendant if they have a procedure to handle this kind of sightings. He told me there is no such procedure and that I should go to inform the civil protection office.
Due to a lack of enough time before the next flight, I asked a security officer to inform the civil protection office, giving her the flight number, exact time of the sighting and after calling the person in charge, she asked me for my name and details about the sighting. Trying to be descriptive, I clearly told her all details before running to catch the next flight and almost losing it.
How should passengers report SOS signal sightings?
P.S. The place of sighting was somewhere over the mountains in the west of Los Andes in Chile.

You did exactly what I would have done as well. Inform the cabin crew. In turn, they can inform the flight crew who will notify ATC. ATC has a direct line to search and rescue.

How is this opinion based?  There are laid down regulations and procedures for dealing with exactly this situation.

Did you eventually find out what happened that day? We're curious. ;-)

@Simon there are procedures for how flight crews should react. As far as I know, there are no procedures for how passengers should react.

@J.Hougaard No, there aren't since you cannot educate and train the entire  population on how to react.  However, there is for all crew and the cabin crew member in this case was wrong to state that there is not.

@Simon Exactly ... which is why a question about how passengers should react is opinion based :)

@J.Hougaard: You're right in many countries. But in some countries it's not a matter of opinion, there are regulations ("In some countries, there exists a legal requirement for citizens to assist people in distress, unless doing so would put themselves or others in harm's way. Citizens are often required to, at minimum, call the local emergency number"). But this is not an aviation topic.

thank you for all your horizon expanding comments :D @PerlDuck on next day i tried to contact the civil protection office and googled for news about alpinists in chile without finding something after some days :/ i still having curiosity after two years and would like to know what happened?

@ncomputers did you ever learn what the issue was?

There is a procedure, at least for the aircraft crew.
It is appropriate for a flight crew to transmit a "pan pan" or even a "mayday relay" so a passenger should immediately inform the senior cabin crew member, usually by asking to  speak to the "purser".  SOS is an internationally recognised distress signal.  If the purser did not act upon it, I would politely, but assertively request that they inform the captain since these procedures are part of standard pilot training and flight crews are required to respond to potentially life threatening situations.
Reporting emergencies or urgent situations is not restricted to the aircraft itself.  It can, and should be used, where any threat to life or safety  is real or possible.  ATC in turn can contact the appropriate local resources depending on the situation.
An example would be if I was flying and witnessed a fire or perhaps a road crash and emergency services were not already in attendance, I would not hesitate
to contact ATC and be prepared to offer assistance if possible, for example relaying position or exact circumstances.
You did the right thing.  Perhaps you could have been a bit more assertive at the time, when the cabin crew member incorrectly told you that there is no procedure, since minutes can count.

"Pan pan" seems to give privileges to the cabin crew, more attention and more resources (runway, airspace...). I see it as a right granted to the crew, not an obligation for the crew to report something to ATC. For the "mayday relay" which seems more obligation related, is it applicable to a flight attendant? By which regulation? In which airlines?

@mins What is the cabin crew going to do with a longer runway or less airspace restrictions? :P

@reirab: My question is how the mayday relay would be known by the flight attendant and force them to transmit the OP observation to the pilot. My comment about the pan-pan was that it didn't look like an obligation to the flight crew (and I wrote cabin crew instead).

@mins Yeah, I figured you meant flight crew rather than cabin crew for that part. :)

@mins.  I don't believe that a passenger could "force" a flight attendant to do anything.  As I say, if I was in this situation, I would politely but assertively ask them to inform the captain.  I know this will vary a lot.  I am a typical alpha male and I know how to be very assertive without being threatening but the first part of this chain is, as you correctly say, not mandated.

How should passengers report SOS signal sightings?

You did it right, the head of the cabin crew must inform the captain, as you asked for, the captain must inform ATC and ATC the rescue organization when the information is credible. These matters are highly subject to judgment at any stage of the reporting chain, and it would be interesting to have a view from a legal person about precedents.

I asked the flight attendant if they have a procedure to handle this
  kind of sightings. He told me there is no such procedure and that I
  should go to inform the civil protection office.

That's probably wrong, the attendant should report this to the captain. Not doing that is probably a professional misconduct. As for the informed captain not complying, they would be exposed to legal action if it was an actual distress situation (see below).
In addition to this crew obligation, anybody can be, to a variable extent depending on the jurisdiction, liable to a duty to rescue. An SOS signal is possibly a sign of distress, it triggers this duty to rescue.
1. ICAO Chicago Convention
The ICAO Chicago Convention is an international treaty, a law for the signatories, implemented in the State legal system by the aeronautical information publication.
The Convention includes Annex 12, dealing with Search And Rescue and its article 5.7 providing a guidance for managing the interception of distress signals. (In aviation terms, the pilot-in-command is the person among the flight crew, legally responsible for the flight, like the captain of a ship):

5.7 Procedures for a pilot-in-command intercepting a distress transmission
Whenever a distress transmission is intercepted by a pilot-in-command
  of an aircraft, the pilot shall, if feasible:
a) acknowledge the distress transmission;
b) record the position of the craft in distress if given;
c) take a bearing on the transmission;
d) inform the appropriate rescue coordination centre or air traffic
  services unit of the distress transmission, giving all available
  information; and
e) at the pilot’s discretion, while awaiting instructions, proceed to
  the position given in the transmission.

It should be noted this article is connoted to rescuing ships/aircraft, which was the concern at the time it was written, but is expressed in such a broad way it becomes applicable to any distress message.

2. Example: Implementation in the French Air Regulations
In the French AIP for La Réunion and Mayotte Islands, those articles are relevant:

GEN 3.6.5.11: Rules to be observed by aircraft commanders
1) Pilots who witness a situation in which people are endangered:
  When an aircraft commander of an aircraft witnesses a situation in which
  people are endangered, he shall proceed as indicated below unless it
  is impossible to do so or, based on the prevailing circumstances, he
  considers it is unnecessary to do so: [...] notify or have someone
  notify, as appropriate:
  - the area control center (ACC), which shall in its turn notify the ARSC [...]
  2) Informed of a:
  a) distress message or signal When an
  aircraft commander intercepts a distress message or signal, or is
  informed by any means, he must proceed as follows in compliance with
  applicable telecommunications procedures [...]
  b) request for
  assistance by message or visual signal: When an aircraft commander
  intercepts a request for assistance by message or by a visual signal,
  or learns of such a request from any source,he shall notify the
  appropriate control or search and rescue unit as soon as possible
  [...]

3. Civil and/or criminal laws
At the State level (for an aircraft this is the country of registration), there are civil and/or criminal laws related to assistance: If someone asks for assistance when in danger, and a person denies it without a good reason, the latter is liable of failure "to assist a person in danger" or to the "duty to rescue". The idea is someone in great distress has a right to be rescued which automatically give others obligations to not abandon them.
In many countries these obligations are relevant of civil code, meaning one can be required to pay damages, but there is no criminal law associated. In some countries, this is a criminal law, e.g. in France not trying to help someone in imminent danger, and when this is possible without endangering oneself, can be punished of a five-year prison sentence. This would apply to any person, witnesses, cabin crew and cockpit crew, ATC, etc.

Sorry, I  don't agree with you on this one.  Since the OP was in an aircraft, and the procedures to deal with this situation are laid down in regulations and procedures, then it is about aviation.

I'd be happy to adjust the answer, but I'm not sure regulations associated with the flight crew are applicable to the cabin crew (and indeed they are not applicable to the passengers). Can you provide details?

Cabin crew are trained and required to act upon, safety concerns from passengers. Here's one example. Performing emergency preparations according to the Pilot-in-Command and/or special instructions; Reporting to the Pilot-in-Command, manage all incidents/accidents, safety concerns of
fellow crew or passengers and situations affecting the safety of the operation

@Simon: IATA isn't a regulating body (and they acknowledge this:"[Nobody] should act on the basis of any such information without referring to applicable laws and regulations*"). Anyway and as the document is 182 pages, could you develop a reasoning based on your knowledge of the relevant articles, that shows the cabin crew was wrong in the OP case?

An important immediate action for a passenger is to note the exact time they saw the distress signal (or start a running stopwatch on a cellphone to keep track of elapsed time) and to note the direction relative to the airplane fuselage. If you can note another direction and time a couple of minutes later, so much the better. With this information and the radar or ADS-B records, search & rescue teams have a much better chance of determining the location.
Unfortunately by the time you've gotten a flight attendant's attention and they've informed the flight crew (if they're even willing to do so) the spot on the ground will probably be out of view, so without a pretty good estimate of the time you saw it the uncertainty in the location will be many tens of km - next to useless for search & rescue purposes.


text:
Enemy plane detection
Suppose that an enemy plane comes into our radar surveilled airspace, and that this plane is mimicking the signature of a friendly aircraft.
Can this tactic be recognized? Is there a way of counteracting it?

It is really unclear what you are actually even asking here. There seems to be some assumptions in your question which may not be true, but it's hard to tell because of how vague it is. If you can give more details, it may be possible to give an answer, but in its current state, there is really no way for anyone to give a clear answer.

What do you mean by "mimicking the signature of a friendly aircraft"? One thing you should look at is IFF systems (Idenfity Friend or Foe): see also this question.

@DavidRicherby that is my interpretation of the original question, see edit history

There are anti-aircraft radar that measure the characteristics of a target by advanced signal processing. But I don't imagine any military is going to ignore a potential threat purely on the basis that it looks on radar like something else - so I don't know of any great effort to produce false signatures. Normally air forces concentrate on making planes less visible to radar.


text:
How are thermals found?
As far as I understand glider pilots like to look out for thermals. I assume that when you are in one, you can perceive that and you can make roughly sure to stay in it.
But what if you're not in a thermal? How do you look for one? Are these methods (like finding a cumulus?) significantly/statistically more efficient than just randomly browsing? Or is it mostly a question of luck?

you look for sunlit slopes and other geographic markers

In addition to cloud formations (cumulus) you can look for ground vegetation (open fields have different characteristics w.r.t. woods, for example)

ratchetfreak and @federico. Yes, but my point is that these seem somewhat like "conventional wisdom" and theorising. Is there hard evidence or really strong theory for these methods to work (and, rather importantly, randomly looking not to work as good)?

The only real way to guarantee the lift is if there is something/somebody else climbing in it...
Hence why often glider pilots don't make it back and end up in a farmers field...  we know where the lift "should" be, and I could tell you, 99 times out of 100, if you have air blowing against a hill it will go up. However, due to the nature of the weather, there is no guarantee that the lift will be there, mountain wave in the right position will often negate ridge lift (which is a bit of a pain if you're not close to the site, personal experience!)
Source: I'm a glider pilot.

There are a few different things that you can look for, and to some degree it is trial and error as well.  Gliders will have an instrument that measures vertical speed so that they can tell "how good" a thermal is as well (a variometer or a Vertical Speed Indicator(VSI)).
Some of the things to look for:

Color variation on the ground because the difference in the amount of heat absorbed by the ground varies based on the color/composition.  The hotter area will cause the air to rise (hot air rises) and will create a thermal.  I.e. A dark colored area surrounded by a light colored area or vice-versa such as a parking lot with fields around it.
Birds circling in an area because they tend to circle in thermals.
Certain types of cloud formations that can indicate lift.
Other gliders!  :-)
If the wind is blowing across a mountain ridge or other large structure, the wind has nowhere to go but up so is also a good place to look for lift as well.

Here is an example showing how some of the thermals form and how the wind affects them.  Notice also how once the air has been lifted to the point where it cools below the dewpoint that a cloud will form:

Here is another showing how terrain can cause lift:

+1 for birds and other gliders. I would like to know if the other bullets have been shown to work better than not following them. (Are these definitely beyond merely theoretical?)

@GlenTheUdderboat Absolutely.  It's pretty simple physics actually.  Think of walking barefoot on a concrete sidewalk compared to an asphalt road right beside it.  The road will always be hotter (when it is sunny) because it is black and absorbs more heat, and will therefore heat the air immediately above it which will then rise because it is hotter than the surrounding air.  Also, certain clouds are only formed by thermals so they are a clear indication of lift.

That is precisely what I mean by 'theoretical'. :) I'm not looking for the explanation, I'm rather hoping for 'experimental evidence'. But because it is 'conventional knowledge', perhaps this is too much to ask for. :(

That isn't theoretical, it's a pretty simple physical process.  Fact:  Dark colors absorb more heat than light colors.  Fact: A hotter area will heat the air above it.  Fact:  Hot air rises.  It isn't my opinion.  :)

I'm familiar with (some) physics. :) But the effect may spread out (and become statistically insignificant) at greater heights. Although, perhaps, the cloud formation mechanism would counter that. (Ideally, I would like to see 'direct' evidence for optimality of such strategies. But that's only ideally.)

I can understand wanting a more technical answer.  I'll try to come back to this when I have more time, but as a glider pilot myself I can assure you that these are very strong indicators of lift and have a high correlation based on personal observations.

As a glider pilot, have you ever been surprised to find yourself in a thermal when you aimed to avoid any thermal? An answer to that would have some significance.

Sure, but that's normally because a thermal is moving with the wind and don't always see the source (especially as the wind gets stronger and moves them further).

That, again, seems (to a layman) like theorising. How can you tell the difference between the wind moving the thermal in your way, and the thermal already being there?

I'm not sure how to respond to that.  Are you implying that the air may actually remaining motionless and stay in one spot even though there is wind?  A thermal moves along with the airmass that it is in just like a cloud does or a boat moves with the water in a river that it is in....

I would assume that a glider usually goes with the wind and not with the ground. But I don't know what I'm talking about. If you care, continue in chat.

@GlenTheUdderboat Sure, join us in [chat].  I'm at work so may be a little slow though.  :)

As a paraglider pilot who has spent hundreds of hours soaring on hillsides, I look for thermal trigger points on the ground and corresponding cumuli forming, then extrapolate the course that the thermal will be following between them.  This is a very reliable method, and on frequently flown hills ‘house’ thermals will be identified that can be reliably found under known conditions.  I’m in no doubt that the underlying theory is correct, but admittedly I can’t usually see what the air is doing (which is as well, as I’d probably never fly if I could).

Furthermore, Baysean logic requires that we try the boot on the other foot, so to speak.  It’s very well established from experience, although probably not well documented, that a glider pilot who stops concentrating in finding lift, often because they are concerned about maintaining a safe landing option, usually ends up on the ground and seldom makes a low save by stumbling on unexpected lift.  Obviously there are other factors at play.  In addition, inexperienced pilots who have no idea now to look for thermals perform much worse than those who do.

the wiki article you linked say the following

Thermals are often indicated by the presence of visible cumulus clouds at the apex of the thermal. When a steady wind is present, thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by soaring and glider pilots.

(source the next linked article)
in absence of humid air (a blue sky) you can still find thermals by the techniques in this article

If the air is very dry or the lift doesn't go high enough, lift does
  not generate clouds to act as markers. On such "blue days" (i.e., the
  sky is only blue, no white), you use other techniques:

"House thermals" are locations that frequently produce thermals. Most local pilots know the location of these faithful life savers, use
  them, and will share the knowledge with visitors or new comers.
Soaring birds have an uncanny ability to find thermals. If you see a soaring bird, following it will usually lead you to lift. If the
  bird leaves a thermal, follow it. It probably senses an even stronger
  thermal nearby. We don't know how birds find thermals, but they are
  better at it than any human or instrument we have yet devised.
Look for other sailplanes that are climbing. I emphasize the last part because several glider pilots often land in the same farmer's
  field. When they discuss what went wrong, each will often say, "Well,
  I saw you circling so I figured there must be lift."
Look for dust devils [mini sand tornado]. The lift can extend well above the top of the visible portion.
Look for topographic features that are likely to trigger thermals. In addition to those enumerated earlier, rocky slopes facing
  the sun, steep canyons, mountain peaks or passes, dark cirques facing
  the sun, and airports (lots of asphalt to soak up the sun's energy)
  are good examples.

+1 for birds and other sailplanes. Are the others more than merely theoretical?

@GlenTheUdderboat According to the glider-folks I've listened to "cloud highways" are definitely more than "merely theoretical" (though not being a glider person myself I can't attest to it firsthand :)

@voretaq7 I don't deny their existence. I only wonder how sure we can be that they can be spotted (other than by birds or other planes using them).

Because they have a line of cumulus clouds topping them. They really are very obvious when you know what to look for.

I once had an instructor who could see thermals against the sky. His vision was acute enough, or developed to be, that he could perceive the shimmer that rising heat produces, distorting the sky behind, or if he was a loft, presumably against the more solid scenery.
Birds on average have about 10 times the number of receptors in their eyes, thus they must see the shimmer 10 times better than the best trained human.
It will be probably possible to produce an AI based detection system designed on that premise, to compare the shimmer of moving air and when it is consistently vertical, to present is on a screen.
Purists would say that is cheating. I'd say, using airplanes is already cheating. Trying flapping your arms instead. We can't tell heights well with our eyes, we can't measure our air-speed, and without a vario most of us would have very short flights. So, using yet another instrument to help us "see" is not any more a copout than wings, rudders and stabilizers.
Such instruments can be disallowed in purist competitions, along with the vario, the altimeter and the ASI, in addition to air-tow, winch, etc.


text:
How do airliners facilitate in flight calls for mobile phones?
How does an airline facilitate in-flight calls for mobile phones?
I would guess that over open water in flight calls are relayed by an on board link to a communications satellite.  Such a system would likely need to track the location of the aircraft.  The records from this system could provide tracking information that is not available from other sources.

See this answer to another question, it covers it: http://aviation.stackexchange.com/a/2022/2

Cribbing liberally from Manfred's answer on a related question, it basically works like this:

The airline contracts with a communication company and installs satellite transceivers and "small cell" cellular equipment on their aircraft. Your phone talks to the on-board cellular equipment just like it would to a cell site on the ground, but the information is relayed to the satellite and sent back to earth to join the regular telephone/data networks.

It's not generally feasible to use these datalinks to track the location of the aircraft -- getting your basic position (latitude/longitude) requires triangulation from at least 3 satellites, while data communication only requires one (satellites are expensive, so providers aren't likely to launch their own constellation of satellites to duplicate the functionality of GPS).
This same type of data uplink could theoretically be used to transmit flight data (including GPS position that the aircraft can get from the existing GPS equipment on board) back to the airline's headquarters, ATC (to fill gaps in RADAR coverage), etc., but I'm not aware of any system or plan to implement that currently.


text:
How is a wing designed to be stall or spin proof?
Yeah, it sounds like two questions, but I suspect the answers are very closely linked (or maybe even have the same answer.)  I was told in this question that trainer planes are often designed to be stall or spin resistent.  I was wondering how this is accomplished?
How does one make a wing resistant to stalls or spins?  Or perhaps, more generally, how does one make a wing predictable and stable?

It's really two separate questions. In both cases the wing is not alone, it is the whole configuration, especially when it comes to spins.
A stall is unavoidable. At some point the airflow over the wing will start to separate, and if this flow separation is extensive enough to limit the increase in lift with increasing angle of attack (AoA), the wing stalls.
To make stall characteristics benign, the outer wing should still have mostly attached flow to enable the ailerons to correct roll angle. If the stall starts on one wingtip, causing a local loss of lift, the aircraft will roll uncontrollably. The downward movement of the wing tip during the rolling motion increases angle of attack further, thus making the stall unrecoverable. You achieve roll control by using washout at the wingtips and/or by using airfoils with a higher maximum AoA, eg. by using slats on the outer wing.
The second condition is a gradual flow separation, starting from the trailing edge. Old five-digit NACA airfoils had a nasty stall characteristic with flow separation starting from the leading edge, resulting in a sudden drop of lift. This is achieved by designing the upper surface with an appropriate chordwise pressure distribution. Then the loss of lift over AoA will be gradual, giving the pilot the opportunity to recover easily.
In all cases, the tail (or in canards the main wing) needs to remain in the attached flow regime to enable pitch control and pitch damping. In addition, the vertical position of the tail should be slightly lower than the wake from the separated flow, still close enough to have some turbulence hitting the elevator (so that the pilot feels the stall with the stick/yoke), but low enough that the airplane will not enter a deep stall (where the wing is fully separated and the tail in the wake of the wing, which reduces control authority to a point where the pilot cannot pitch down anymore).
Spinning needs a fuselage with some mass along the length. The rotation of this mass at a high AoA produces a pitch-up moment which is needed to stabilize the spin. The tail must be able to produce the remaining pitching moment and a yawing moment to stabilize the spin, but also to end it when the pilot so wishes. For this, it is important that the rudder is not in the wake of the horizontal tail. The deHavilland Tiger Moth has two aluminum strakes ahead of the empennage; without them a stall is unrecoverable because the rudder authority is not sufficient. To end a spin in the F-14, the all-flying tailplanes had to be pulled all the way to their minimum angle of -70°, so they would not obstruct the flow to the two rudders. Only when the pilots would pull, they could end the spin.
Just to be clear: Normally you need to push to end the spin. Especially in gliders with their relatively small rudders and large wing inertia, this will put most of the wing back into the normal flow region, and roll damping will stop the rotation. This ends the inertial pitch-up from the fuselage, and the aircraft can be recovered.
In more fuselage-dominated configurations, you end the spin by applying rudder against the spin direction. This reduces the rotation, reducing the inertial pitch-up and allowing the airplane to recover to normal AoAs.
Other than the stall, a spin is not always possible. Sometimes, especially at forward center-of-gravity locations, the pitch authority will not be sufficient to stabilize the spin. You can stall and apply rudder, but the airplane will only enter a spiral dive. Especially on jets the forward fuselage is a major factor in spins, because it produces a wake at high AoA which stabilizes the spin. Details in the shape of the fuselage will determine if the wake is sufficient to enable spins. This is a complex topic - sorry, but this scope makes it hard to get into more details.

There are several ways a small airplane can be designed to be resistant to stalls and spins. Both cases are somewhat similar in that they both involve cases where the aircraft's controls are rendered ineffective, and a stall can lead to a spin if not properly corrected. While a stall is certainly largely dependent on the aircraft's wing, both stalls and spins also depend on the whole aircraft's geometry.
There is some interesting information here about spins and stalls. Wikipedia also has a good article about spins. There are FAA requirements for single-engine planes in FAR 23.221.
In order to maintain control in a stall, devices like vortex generators can be installed upstream of the ailerons. If the air flow separates from the wing before the ailerons, the air is no longer flowing over the control surfaces, resulting in a loss of control. The vortex generators help the flow to remain attached past the ailerons, helping them to stay effective at high angles of attack.
Besides vortex generators, the wing itself can be designed for a similar effect. The inboard sections can be designed to stall before the outer sections, so the lift decreases but the outer sections will still provide lift and control.
The aircraft can also be designed such that the elevator does not have enough authority to fully stall the plane, or not any further than the ailerons will still have authority. The effect of the elevator will also depend on the airplane's center of gravity, which has a suggested range but is determined by the operator.
A spin is related to the geometry of the airplane, where the air flow past the wing "blanks" the tail surfaces. This geometry can be arranged such that it would be more difficult to create spin conditions. However, conventional aircraft designs tend to inherently be susceptible to this. Most aircraft are certified as "spin recoverable", meaning that the aircraft will spin, but the pilot should be able to stop a spin with the proper control inputs. Improving the plane's stall characteristics will also help prevent spins from happening.

Nice, very good answer.  I do wonder, when a wing is fully stalled, will the vortex generators also aid in stall recovery?  It stands to reason...but I thought I'd ask.

Depends what you mean by "fully stalled." Of course the vortex generators are only effective up to a certain angle of attack, after which they won't matter as much. But increasing the controllable AOA will certainly help throughout at stall and recovery.

The control surfaces are still working, because the flow stays attached to the lower part. They are less effective, yes, but not ineffective. In bad designs the upper flow separation becomes so much worse with aileron deflection that the rudder authority is reversed, but such bad designs should be older than you and me together.

One point to consider is that even if an elevator doesn't normally have enough authority to fully stall the plane in a steady state, the plane could still enter an accelerated stall by pulling back abruptly at high speeds.

You may want to look at NASA's spin research program, appropriately described as as a 'spin-off'. There is a complete document on the web somewhere I post it if I find it. The knowledge gained from the research was actually incorporated into newer designs.
The Ercoupe was built as a safety plane that could not stall or spin, however there have been several crashes involving the plane, as accident records show.


text:
How does chord length affect wing design?
In comments on the answer to a question about transonic speeds and control surfaces I came across this comment and I'm having a hard time understanding it.

If you increase the chord, lift goes up almost linearly and stability
  decreases, because now the wing's lift force has a longer lever arm in
  flow direction. By increasing chord, you decrease aspect ratio...

There are a few things I am not understanding, but I think they probably relate quite heavily, so I'm going to ask them together.
Firstly, what is the aspect ratio of the wing?  Is that a reference to the incidence of the wing and how a longer wing would mean a larger percentage of the side of the fuselage is covered by the wing (as seen from the front)?  Or am I way off the mark?
Secondly, why does increasing the chord length decrease the stability of the wing? In my mind a longer wing would mean that the craft is more stable because the airflow is being forced into line for a longer period of time.
Lastly, this all relates to how chord length is considered during wing design.  Are there other factors centered on the chord length that should be considered when designing a wing?  And do those factors change depending on the mission of the craft in consideration (eg., trainer v. cargo plane v. jumbo v. Mach 3 spy plane, etc.)?

For aspect ratio, see: What dictates the aspect ratio of an aircraft's wing?

I'm no aero engineer but my gut tells me a longer chord decreases the stall angle of attack

@fooot Thanks for the link!  @ratchetfreak() I guess we'll see, Peter Kämpf is making me feel like he's going to put some great answer up for this question so... we'll see :)

@ratchetfreak lol, sorry, the second half was meant for you.  The first half for fooot...

During design, chord length will be the result of wing span and wing area, because those two are what the designer wants to set. To be more precise, he tries to set a certain wing loading, trying to minimize wing area (and weight) such that the wing creates just enough lift for the estimated aircraft mass in all design cases. Take-off, maximum altitude and minimum turn radius are the classics which drive minimum wing area.
Now I will focus on aspect ratio, because Jan has already answered the rest with his excellent answer. The higher the aspect ratio, the less the wing tips will influence the airflow around the wing. This means that the wing creates more lift for a given angle of attack, but also that the stall angle of attack is lower. The ratio of wing span to aircraft mass (called span loading) is the driving force for induced drag. However, wing span and aspect ratio will also drive wing structural mass, so you need to find a sound compromise between low induced drag and low wing mass.
What is induced drag? It is the consequence of creating lift over a limited span. The wing creates lift by deflecting air downwards. This happens gradually over the wing's chord, and creates a reaction force orthogonally to the local speed of air. This means the reaction force is pointing up- and slightly backwards. This backwards component is induced drag! The wider the wing is, the more air can be used for lift creation, hence less deflection is needed. Consequently, the backwards tilt of the reaction force is smaller, resulting in less induced drag for the same lift.
If you fly fast, there is a lot of air mass streaming past the wing per unit of time, so you need to deflect the air only slightly. Your induced drag is small. That is why induced drag changes inversely with air speed.
Now you know that for high speed at high density, induced drag is not important. If you design an attack aircraft which has to fly at low altitude, a low aspect ratio will help: The lift increase due to gusts is smaller than with a high aspect ratio wing, and the induced drag is manageable.
How the lift curve slope changes with aspect ratio in subsonic flow is shown in the simple plot below. For a slender body (aspect ratio $\approx$ 0), the gradient of the lift coefficient $c_L$ over angle of attack $\alpha$ is $c_{L\alpha} = \frac{\pi \cdot AR}{2}$. Please note that the red line is only valid for AR = 0! Then the lift curve slope increases up to $c_{L\alpha} = 2\cdot\pi$  for $AR = \infty$ (and zero airfoil thickness and no friction effect), as shown by the blue line.

There are more influences, however. Dihedral means that the lift is tilted inwards, and the part which counteracts weight is only growing with the cosine of the dihedral angle $\nu$. Same goes for sweep: Sweeping means that the wing only sees a reduced angle of attack change. If you want to capture all effects, a plot will not suffice. Here is a table with formulas for most cases:

Nomenclature:
    $c_{L\alpha} \:\:$ lift coefficient gradient over angle of attack
    $c_{L\alpha\:ic} \:$ lift coefficient gradient over angle of attack in incompressible flow
    $\pi \:\:\:\:\:$  3.14159$\dots$
    $AR \:\:$  aspect ratio of the wing
    $\nu \:\:\:\:\:$  the wing's dihedral angle
    $\varphi_m \:\:$  sweep angle of wing at mid chord
    $\varphi_{LE} \:$  sweep angle of wing at leading edge
    $\lambda \:\:\:\:\:$  taper ratio (ratio of tip chord to root chord)
    $(\frac{x}{l})_{d\:max} \:$  chordwise position of maximum airfoil thickness
    $Ma \:\:$  Mach number

Another consideration is wing volume: In most aircraft, the wing will hold most of the fuel, and a long-range aircraft needs big tanks. Sometimes, only choosing a lower aspect ratio will give enough wing volume for the required range. In order to keep induced drag constant, the span will be kept the same, so wing area increases with chord length. This has the added benefit of more lift, so less complex high-lift devices are needed. Since the structural mass of such a low aspect ratio wing with simple flaps is relatively low, the only disadvantage is the higher friction drag of this larger wing.

If maneuverability is important, wing span needs to be as small as possible. This reduces inertial moments and roll damping, so the aircraft can accelerate faster into a rolling motion and will reach a higher roll rate. This is extremely important in dogfights when the one who points his radar, gun and rockets at the adversary first will win. Here the chord is chosen for sufficient wing area at the minimum practical wing span.

Now we need to talk about viscous effects. Friction between air molecules and between air and wing. The ratio of inertial to viscous forces is expressed by the Reynolds number, and generally a higher Reynolds number means your friction effects go down, translating into less friction drag and a higher stall angle of attack. Especially for model airplanes, but also for gliders, it is sometimes better to reduce aspect ratio in order to gain absolute wing chord. The Reynolds number increases linearly with wing chord, and this can also be a consideration for choosing wing chord.

A normal wing has positive camber, and this means that the center of pressure moves forward with increasing angle of attack. Moment-wise, this means that the wing will create a stronger pitch-up moment when it pitches up. This makes the wing by itself unstable, and you need a tail surface to regain stability. Making wing chord longer will increase the unstable influence of the wing relative to the unchanged tail surface. This is why I said that increasing chord will decrease stability. During design, you watch for a size called tail volume. This is the area of the horizontal tail surface, multiplied with it's lever arm expressed as a multiple of the wing's reference chord. If you keep this constant while changing wing chord, your stability stays the same. But then you change your pitch damping, because this is affected by the square of the lever arm. Digging deeper will uncover more consequences, so I better stop here for today.

Every time I read one of your answers I feel like I should just bite the bullet and get a masters in fluid dynamics/wing design.  I may have to read this another dozen times before I get it, but I'm enjoying trying to decipher it as best I can :).

I'm going to need to write up a question on what the horizontal tail surface is, it would seem.  I can see how you'd need to counter-act the increased moment created by the center of lift moving forward with a positive angle of attack...but I'm not quite understanding how it's done.

Yes, please go ahead and ask. To answer this in comments is not as easy.

What do the dotted lines represent in the wing planform pictures?

@DrZ214 The quarter-chord lines.

Aspect ratio is defined as
$AR=\frac{b^2}{S}$
where $b$ is wing span and $S$ is wing area. For rectangular wing that's the same as span to chord; using area in the definition side-steps the need to define average chord for more complicated wing shapes.
Some properties of the wing depend on it's area while other depend on the span:

Lift is proportional to area and square of indicated speed (dynamic pressure).
Induced drag is inversely proportional to span and inversely proportional to indicated speed.
Form drag is proportional to span, thickness and square of indicated speed.
Skin drag is proportional to area and square of indicated speed.
Wave drag is proportional to span, thickness and grows rapidly above drag divergence mach number.
Strength is inversely proportional to span.

For each wing there is an (indicated) speed at which the wing is most efficient as induced drag has reduced already and the other forms of drag didn't yet grow too large.
Since induced drag decreases with span, high aspect ratio wings have the optimal point at lower speeds and their total drag at that speed is generally lower. That's why gliders have wings with very high aspect ratio (very long). As the other forms of drag increase with span, for higher speeds, especially supersonic, lower aspect ratio is better.
Note, that all the speeds are indicated speeds. Indicated speed is dynamic pressure expressed as speed at which it occurs at sea level. But as density decreases with altitude, so does indicated speed. Therefore transport jets cruise at modest indicated speeds and can have relatively high aspect ratio wings.
The other consideration is strength. Making long wing strong is difficult, so aircraft that need high manoeuvrability (aerobatic and fighters) can't have wings with high aspect ratio.
As for stability, during normal flight position of centre of lift along the wing chord is relatively stable. But there are three conditions that change it:

Stall. Pre-stall lift has centre of pressure about quarter chord, but post-stall lift has centre of pressure midchord. The resulting down pitching moment is a good thing in this case.
Mach tuck. This is flow separation caused by shock wave forming on the upper surface of the wing as the aircraft approaches speed of sound. The effect is the same as stall, except this time the down pitching moment is bad thing, because further increase of speed is not desired in this case. Supersonic aircraft usually have all moving horizontal stabilizers to have enough elevator authority to compensate for this change.
Extension of flaps. These also shift centre of lift more aft and create down pitching moment.

All these effects will be more pronounced in long chord - low aspect ratio - wings.

Excellent, thank you, great explanation of Aspect Ratio...had to read it 3 times, but I think I've got a much better grasp of it now :).


text:
Despite the lessons from DC-10 planes in 70s, why didn't Boeing ensure their cargo doors didn't fail even until the end of the 80s?
In the early 70s, DC-10 planes had disastrous failures with cargo doors. It proved how massive a havoc faulty cargo doors can have on the planes. Despite those two incidents with Boeing 747 planes in the 80s prove that Boeing were not careful enough to avoid problem with the cargo doors. In fact, there is a striking similarity with what happened to DC planes in 1972(a warning) and in 1974(not learned from the warning) and what happened to Boeing 747 in 1985(warning) and in 1987(not learned from warning).
Why didn't Boeing learn from what happened to DC-10 planes?

"Why didn't Boeing learn from what happened to DC-10 planes?". The answer will be clearly an opinion, as I doubt anyone can know what were the decisions taken at the Boeing management level, and why they were taken.

Most probably because the failure modes were very different. In the DC-10 case the door could appear closed even though the locking mechanism was not properly engaged. On 747 that was not a problem; instead the electric motors opening the door had enough power to break the locking mechanism if electric power somehow made it around the switch that was part of the locking mechanism.
And then, 2 years are not really enough to analyse a problem like this, develop a fix, test it and deploy it. The NTSB investigations alone often take longer than 2 years.


text:
Why is C-5 Galaxy not used for commercial airlines while Boeng 747 is?
For a layman like I am, both the planes look massive and similar in size and shape, as if a near copy of each other. Do the similarity only end in the size and shape? Do they not perform equally well in similar roles? While Boeing 747 is used for commercial airlines, why isn't Lockheed C-5 Galaxy? Similarly, why is C-5 galaxy popular for Military and 747 isn't?

Even though it is not popular, the B747 has been used for military experimentation (YAL) and for advanced command post (E4)

Another similarity is the origin: Both are the offspring of an Air Force contract from 1963 for the development of a large cargo aircraft (CX-HLS). Boeing lost and turned its entry into the 747. This is the reason for the raised cockpit of the 747, and also the engines for the 747-100 originated from the CX-HLS development.

@ManuH The 747 also has another somewhat more well-known role in the USAF. And it looks like it will continue to have such a role for the next few decades, at least.

C-5 Galaxy was designed to military requirements that for cargo aircraft include:

operating from unpaved runways (I am not sure in case of C-5, but probably yes)
operating from airfields without much support
operating from short runways
paradrop capability

These basically imply a high-wing design so that engines are high (to minimize risk of foreign object damage) and the deck is lower for easier loading, front and rear cargo door with loading ramps so loading is possible without any ground infrastructure and some excess thrust for sufficient performance on short fields.
However that design is less efficient. It has higher operating cost. The air force is willing to pay it, because it needs the capabilities it requested. But commercial operations don't need those things, because it has enough well equipped airports to operate from. So the higher cost of operating an L-500 (as civil version of C-5 would be called) makes it uninteresting.
B-747 only shares one design feature with C-5, the raised cockpit that allows full-height (well, almost) cargo door in the nose, but otherwise it is optimized to minimize operating cost, making it great cargo plane for operations between airports.

On a side note, there is some commercial demand for transporting heavy or large cargo to less equipped airports. These are currently covered by ex-soviet-military An-124 aircraft that look very similar to Galaxy, are a bit larger, are certainly able to land on unpaved runways and were simply available when Soviet Union collapsed.

This is the only answer thus far to mention the actual root answer - the cost of flying it.

Though I could not find the tire pressure of the C-5 online, the number of tires (28) compared to the 747 (18) shows that the C-5 should be able to operate from lightly prepared runways.

@PeterKämpf: Given that An-124 only has 24 and slightly higher MTOW and that I am quite sure An-124 was designed for it, probably indeed so.

The C-5 is not a tactical airlifter - the short field/unprepared field role is not its role, that belongs to the C-17.

@Harper While the C-5 is used for strategic duties, the original design called for short-field capability.

@cpast The C5 never lands on dirt. There is some ridiculous amount of maintenance required whenever it does. I couldn't find the reference to get the actual number.

@Pilothead Like I said, it's actually used for strategic duties (the Air Force doesn't even plan on using it on unpaved fields in wartime). But the initial design called for the capability, and the plane was designed to have it (it just didn't do a good job at it).

Please, put some reference, especially "short runway".

A commercial jet, designed to carry people, like the '747 or any other jetliner, need to fullfill a series of requirement about:

noise (both inside and outside)
safety (emergency exits, oxygen masks and other safety devices)
dimensions (space between gates at airport are a design contraint)

Also, the areas around windows and doors require reinforcement in the fuselage.
All those things are unnecessary on a military transport. Other features are usually required like:

operating from unpaved runway (need special landing gears)
mid-air refueling (too dangerous for a jetliner)
being able to drop cargo mid-air (requires a ramp on the tail of the aircraft)

A lot of considerations can be done, but converting or modifying aircrafts isn't an easy thing.
One big exception are tankers. There's only one KC-747 Tanker and is flown by the Iranian Air Force.

Dimensions aren't such an issue for this particular comparison: the C-5 wingspan is 10ft more than a 747-400 but slightly a couple of feet less than an -800.

@DavidRicherby for this particular comparison you're right. I'd like to point out btw that the '744 has a wingspan of 211 ft 5 in, while the '741/742/743 has a wingspan of 195 ft 8 in. C-5 has a wingspan of 222 ft 9 in: That's 27 feet more than a "contemporary" '742.

Is there reference in your opinion? Especially "safety", I wonder how military design and verify it.

@CloudCho references... about what? About the fact that a military plane designed to move cargo doesn't need emergency exits like the ones of a passenger jetliner and that doesn't need to be certified with, like, evacuation tests?

I searched with "military aircraft evaluation test" and this result, https://www.af.mil/About-Us/Fact-Sheets/Display/Article/104538/air-force-operational-test-and-evaluation-center/, looks close. Do you have any other key word suggestion. By the way, your post is mostly on based on your own idea/experience rather than web search?

@CloudCho, sorry but I think I'm missing the point... maybe due to a language barrier (by my side probably, sorry). In my reply, 9 years ago, I just pointed out that passenger aircraft need to be certified according to a certain set of standards, including, but not limited to, safety standards, which aren't the same as the military ones (like I've never seen a military cargo aircraft, which is not a converted passenger aircraft, equipped with inflatable door slides). Are there safety standards for military cargo planes? Yes, probably some NATO ones. Same as civilian standards? Don't think so.

I doubt any commercial operator would want to use a C-5. The 747-400 series is roughly contemporary with the C-5B. Compared to the 747-400F freighter, the C-5B:

is slower (cruise speed of Mach 0.79 vs Mach 0.85);
has a smaller internal diameter (19ft vs 20ft) and isn't much longer (247ft vs 231ft externally);
has a cripplingly short range when fully loaded (2,760mi without refuelling isn't even enough to get you from London to Moncton, let alone London to New York, vs over 5,000mi);
carries less (122t maximum payload, vs 128t for the 747-400F)
requires a cockpit crew of four, vs two;
uses huge amounts of fuel (the 747 gets twice times the range from fuel tanks that are only 10% larger).

The main benefits of the C-5 are that its high wings and low landing gear make it easier to load: you don't have to lift everything fifteen feet in the air to get it into the plane. and I think the C-5's cargo bay is full-height for its whole length, whereas the 747 has reduced height at the front under the cockpit. That also makes loading a 747 something of a sliding-blocks puzzle, since any full-height load has to go in through the side door and be turned, which limits the length of any item; the C-5 has a tail ramp that lets you load an object of almost any length that will physically fit. The C-5 can also use lower quality, shorter runways.
Airlines, even cargo airlines, don't carry many long, tall, heavy loads, and they fly to and from well-maintained airports with nice runways, so don't need the advantages the C-5 gives.  That just leaves the disadvantages.
Sources: C-5 on Wikipedia, 747 on Wikipedia, Boeing 747-400F factsheet

It's a little unfair to compare the C-5A with the 747-400. There are 30 years of progress between the two (supercritical airfoils and much better engines make the 747-400 look much better). If the C-5 would be re-engined with modern RR Trents and an improved wing, its range and cruise Mach number would be much closer to that of the 747.

@PeterKämpf That's why I specifically compared the C-5B against the 747. Wikipedia describes it as having improved wings and engines over the C-5A.

Yes, but it had still the old TF-39, just upgraded. Also, the wings received only structural reinforcements, and kept their old airfoils. It was what Lockheed had promised the C-5A to be.

@PeterKämpf OK -- thanks for making that clear. I thought it was a more significant upgrade. When I have time, I'll update to compare against earlier 747 models.

While Boeing 747 is used for commercial airlines, why isn't Lockheed C-5 Galaxy?
The C-5 was created on a proposal made directly to the military to fill a heavy lift role. It is more rugged than a 747 and includes things that only military operations call for like countermeasures and probably lot of other classified systems. It's uniquely suited for evasive maneuvers and down range operations that a 747 wasn't designed for. It's specifically a military aircraft with no civilian version.
Similarly, why is C-5 galaxy popular for Military and 747 isn't?
The 747 is popular for military use, but mostly in the form of contract work. The military hires private companies to move cargo with 747s all the time. The military does have some 747s of their own too. The E-4 is one example. And there's always Air Force One.

A VC-25 is always a 747 but Air Force one isn't always a VC-25.

boeing designed a 747 missile carrier

That isn't 100% correct. Air Force One is the call sign of the aircraft of the US Air Force , which houses the president of the United States. I'd rather say: And there's the 747-200B used as Air Force One.".

C-5 contains many classified systems, but so does VC-25. On the other hand, Lockheed did offer L-500, a civilian version of C-5, but no airline showed interest and the project was cancelled. Military systems are not the difference. They can be mounted on about anything. The airframe design is the difference.

In his book 747, Joe Sutter (the project lead for the initial 747) mentions that the idea of using the C5 as a transport was floated by then president Lyndon Johnson as an alternative. The US was experiencing a period of high economic inflation in the mid 1960's, and Johnson was putting the brakes on corporate expenditures so as not to drive prices even higher.
Sutter points out that the C5 simply wasn't economical enough to consider as a civilian transport in a competitive market. Its fuel usage was too high, and it had a slower cruising speed than most civilian airliners of that day.
Being a military aircraft, the C5 is designed to operate under different conditions than civilian airliners... short and/or rough field capability (heavier landing gear), surviving battle damage, etc... All of that adds weight, making the aircraft less economical to meet requirements that it won't encounter in civilian use.
Plus, military transports aren't designed with passenger comfort in mind, as anyone who has ever spent a few hours in the back of a C130 or C141 can attest to. More weight gets added to address that situation, and that means more fuel usage.
With the military, the taxpayer foots the fuel bill, and has no choice in the matter. Nor do the soldiers riding on the transports have any choice. In the commercial airliner field, the customer foots the bill, and definitely has a choice... another airline, flying a more economical and comfortable aircraft. It was the fuel efficiency of the 747, as compared to the smaller turbojet airliners of the 1960's, that made it a great success.
These are essentially the same reasons that the current C17 and A400 aren't used as an airliner. Not as efficient as existing airliners, and designed for situations that commercial airliners won't encounter.
Ironically, the 747 project was considered a sideline at Boeing when it first started up in the 1960's... all of the attention, and the best engineers, were on the 2707 supersonic transport.

The C-5 Galaxy is not for sale
The USAF jealously hoards certain aircraft.  They are shared with foreign militaries only very reluctantly and never Civilians.
Think of the A-10 Warthog (contrast with the Su-25 which Russia sells to pretty much everybody who wants CAS), B-52, F-111 (shared only with Australia, and only near its end of life), F-14 Tomcat (its only share being a complete disaster), and indeed C-5 Galaxy.
Anyway, it's not competitive
A civilian entity seeking a strategic airlifter has so many options.  The 747 is rapidly becoming an aircraft without a market, so they are readily available, and of course Boeing would fall over themselves to sell you a new one.  Meanwhile Antonov would not only be absolutely thrilled to sell you the An-124, they can offer you positively insane pricing because of the odd economics in Ukraine and Russia of late.
As a partner, Antonov totally understands you because they are you - having spent 20 years themselves being a commercial cargo carrier and tackling loads absolutely no one else can carry with their own An-124 fleet and An-225.   Boeing has 40 years of experience specifically in the 747 supporting commercial customers who are most concerned with total life-cycle cost.  Whereas you won't get any love from Lockheed.  Their last venture into the finicky commercial market, the L-1011, was a complete disaster and nearly bankrupted the company.  They don't even possess a sales and support division to deal with customers who are not in Arlington, Virginia and whose offices are not 5-sided, and that will make you a penny-ante annoyance. They'll charge you "I don't want to deal with you" prices.
The commercial win is to stick with 747 unless you want to take chances with the An-124.  Heck, Antonov would love to finish the other An-225 for you, and that's something not Boeing and not even Lockheed can supply.

Lockheed offered a civilian C-5 on the open market. The F-111 was offered to the UK in the 1960s. The F-14 was for sale too. Dunno about the others. The lack of sales was more likely caused by your examples being too specialized and/or expensive for other air forces to operate.